The present invention relates to a method and an apparatus for treating an aqueous medium containing an organic matter. The present invention more particularly relates to a method for preparing a starting slurry from an organic waste in solid, sludge, or liquid form, such as municipal waste, night soil, sewage sludge, industrial wastes and so on; and subjecting the starting slurry to two stages of hydrothermal reactions.
Conversion of waste into fuel or a carbon source has been receiving attention. Wastes such as municipal waste, night soil, sewage sludge, industrial wastes and so on have a tendency to contain an organic matter, and thus it is desired to convert the organic matter into an energy source and a carbon source for so-called Cl chemistry in a form easy to handle. It is further desired that the energy source has a high calorific value.
A hydrothermal reaction has been investigated as a prospective method for converting an organic matter in waste into a valuable resource. Specifically the method includes the steps of: preparing a starting slurry containing water from waste, which may further include the steps of pulverizing waste and then classifying the pulverized waste; performing a hydrothermal reaction onto the starting slurry to give a carbon slurry; and then removing water as well as organic components dissolved therein from the carbon slurry.
FIG. 3 shows a flow diagram of a conventional method having a hydrothermal reaction. Firstly, the waste is converted into a starting slurry. When the waste contains a solid matter, the solid matter is pulverized to give a plurality of solid particles having a diameter up to several millimeters. In the pulverizing step, inorganic solid matter, such as glass, tiles, pebbles, and metal are removed as much as possible. On the other hand when the waste is a sludge, a waste liquid and so on which do not require the pulverization, the waste is, if necessary, passed through screen so as to remove large a solid matter having a dimension larger than about 10 millimeters.
Preferably, the pulverizing step and the classifying step may be carried out in a liquid medium, such as a water medium, having the solid matter dispersed therein so as to facilitate obtaining a starting slurry. Alternatively, the pulverizing step may be carried out in air, and then the pulverized waste may be mixed with a liquid containing water to give a starting slurry. Preferably, the starting slurry has a water content and a fluidity such that the starting slurry can be transferred by means of a pump. The starting slurry typically has a medium containing water and substances dissolved in water and a plurality of combustible particles dispersed in the medium, and the starting slurry has a high viscosity.
Then the starting slurry is subject to a hydrothermal reaction in the absence of the external oxidizing agent to give a carbon slurry. The hydrothermal reaction refers to a reaction in the presence of water at high temperatures under high pressures. In the hydrothermal reaction, for example, the starting slurry may be pressurized to a pressure of not less than 150 atmospheric pressures, and the slurry was heated to temperatures of 250.degree. C. to 350.degree. C. while retaining the pressure for one to two hours. In a continuous treatment, an inside of a reaction vessel may be heated in advance, and the starting slurry may be pressurized to be fed into the reaction vessel so that the starting slurry at high temperatures under high pressures is allowed to remain in the reaction vessel for a prescribed period. In contrast, in a batch treatment or a semi-continuous treatment comprising a plurality of batch treatments, the starting slurry is charged into a reaction vessel which can withstand internal pressures, the reaction vessel is enclosed, and the temperatures thereof are increased so as to increase pressures inside the reaction vessel increases accordingly.
In the hydrothermal reaction, a variety of organic matter present in the starting slurry decomposes. For example, a dissolved oxygen gas and oxygen atoms in the organic matter oxidize the organic matter to give carbon dioxide. A small amount of a reductive gas such as ammonia, hydrogen sulfide, mercaptan and so on may be produced. Moreover, a salt in the waste may be transferred to an aqueous phase. Furthermore, a halogen atom such as a chlorine atom, a bromine atom, etc., contained in the organic matter may be eliminated therefrom so as to form a salt containing the halogen atom, thereby transferring the salt to an aqueous phase.
A carbon slurry refers to a mixture of water, a solid matter, and an oily matter. The solid matter, which may be referred to as a char and the oily matter may be produced by dehalogenation and decarbonation of at least a part of the organic matter. The oily matter herein refers to those which can be extracted by an organic solvent such as hexane, dichlorobenzene and so on. Therefore, the oily matter includes those solidified at room temperature, such as tar and pitch, and an organic compound soluble in water. An aqueous phase refers to a filtrate obtained by extracting the carbon slurry, which contains water, by the organic solvent; and filtering the remaining aqueous layer. A solid phase refers to a solid matter or a cake obtained by filtering the remaining aqueous layer.
After the hydrothermal reaction, a flush operation, indirect heat exchange and so on are carried out so as to reduce the pressures to atmospheric pressure, to cool the carbon slurry and to degas.
The solid matter and the oily matter obtained are substantially free of a halogen atom, have a low water content, have a relatively high calorific value, and are allowed to be easily pulverized.
When the staring slurry has a plurality of combustible particles therein, the hydrothermal reaction gives a plurality of solid particles in the resulting carbon slurry, and the solid particles are fragile char, which has an improved carbon content. The decomposition reaction in associated with the hydrothermal reaction decreases a viscosity of the slurry, and therefore facilitates removal of the aqueous phase from the carbon slurry. The removal of the aqueous phase allows the solid particles to serve as fuel for, for example, a boiler having a fluidized bed. Alternatively, the isolated carbon particles may be pulverized in oil or water to obtain a concentrated carbon slurry, which serves as fuel. This slurry is a pseudo liquid in the sense that it has fluidity, and the form of the slurry is convenient for storage and transportation thereof. The concept is somewhat similar to coal oil mixture (COM) and coal water mixture (CWM), which have developed in view of the utilization of coal. For example, a concentrated carbon slurry having a calorific value of 3000 to 5000 kcal/kg may be obtained.
A cake obtained by filtering the carbon slurry itself may serve as fuel for a boiler having a fluidized bed; and the cake may be fed into a furnace thereof. The cake tends to be in a nodular form due to its moisture, and the cake is fed into a fluidized bed without being flown away by a gas generated in the furnace. The cake is mixed with a fluidized medium having temperatures of 600 to 850.degree. C., dried, collapsed, and ignited (burned) thereby improving fuel efficiency. A so-called return ash process, which comprises the step of feeding into the fluidized bed again precipitated ash in a bottom of the boiler and collected dust ash in a flue bottom, further improves the fuel efficiency.
When the organic matter present in the starting slurry contains a halogen atom, the hydrothermal reaction to form a carbon slurry and subsequent removal of the aqueous phase from the carbon slurry substantially removes the halogen atom. On the other hand, the carbon slurry containing the halogen atom is disadvantageous as fuel. When such carbon slurry is used as fuel, the halogen atom in the carbon slurry corrodes a metal which comes into contact with an exhaust gas, melts the ash to give a scale or clinker and vaporize a heavy metal. Moreover, when such carbon slurry is used as fuel for firing cement, the resultant cement has improved salt concentrations. Therefore, the removal of halogen atoms from the carbon slurry decreases metal corrosion, prevents the formation of the scale and clinker, and enables its application as fuel for firing cement. The term halogen includes fluorine, chlorine, bromine, and iodide. Chlorine, which tends to be present significantly more than the other halogens, is particularly problematic for example when the waste is municipal waste and when the waste contains a chlorine-containing polymer, such as polyvinyl chloride, polyvinylidene chloride and so on.
After removing the aqueous phase from the carbon slurry, the resulting residue may be washed with water for further removing salts containing halogen atoms, and, if desired, water in the residue may be removed to give the solid matter and the oily matter. As an apparatus for removing the aqueous phase and washing the residue, a centrifugal dehydrator having a bucket may be preferably used so as to efficiently perform the dewatering step, the washing step and the dewatering step of the cleaning water.
However, the aqueous phase removed from the carbon slurry contains a high concentration of organic matter, and the concentration thereof typically corresponds to a range of 20,000 to 100,000 PPM of chemical oxygen demand. Such organic matter includes, for example, alcohol, ketone, amine etc. Typically water containing an organic matter undergoes a biological treatment. However, the aqueous phase containing this level of organic matter cannot be subjected to the biological treatment. Usually, the aqueous phase needs to be diluted by a large quantity of water prior to the biological treatment. Alternatively, the aqueous phase is required to be vaporized and oxidized by fuel oil.
Therefore, it is an object of the present invention to provide a method for decreasing the chemical oxygen demand of the aqueous phase from the carbon slurry to such a level to allow biological treatment. Moreover, it is another object of the present invention to provide a method for treating not limited to the aqueous phase from the carbon slurry but an aqueous medium in general containing an organic matter so as to decrease the chemical oxygen demand thereof.